A grating ruler is a feedback-based measuring device relying on the optical principle of optical gratings, and is usually used in closed loop position control systems as a testing apparatus, particularly in the fields of precision processing and manufacturing. The basic principle on which a grating ruler works is that the relative movement between the ruler and its scanning mask forms a Moire pattern under light. The Moire pattern is converted into similar sine and cosine electric signals by a photoelectric sensor. These signals are the original scanning signals of the optical grating. Then, by using different electronic subdivision methods, counting pulse signals with different measuring pitches can be obtained. These pulse signals are generally two mutually orthogonal signals. The two signals are fed into a downstream reversible counter circuit. Then the reading of the counter is multiplied by the measuring pitch to get the displacement value of the grating ruler.
A common way to enhance precision of displacement measurement is to increase the density of the graduation of the optical grating so as to significantly increase the amount of data collected. However, in occasions where both high speed and high precision are desired, while the huge amount of data collected, as mentioned previously supports great precision of the grating ruler, it poses significant limitation to the speed of measurement. Thus, it is a dilemma between high speed and high precision.
The presently common grating ruler mainly uses “Moire patterns”, together with tests such as electronic subdivision and a data processing module to obtain displacement values. The existing grating ruler measuring system primarily has the following problems. For improving the precision for displacement measurement, the grating is desired to be made dense. With the increase of data obtained from the dense grating for each unit of displacement, the risk of “losing steps” during data acquisition increases, which jeopardize the authenticity of data. Particularly, for high-speed motion, the effect of “losing steps” is more significant. Thus, using devices with increasingly higher acquisition speed without deliberation can seriously increase the costs for making and using measuring systems.
China Patent No. CN200510010288.1 provides a method of measuring high-speed high-precision displacement based on two grating rulers. The core concept of the related art is that: a normal high-speed, high-precision positioning system does not have high-speed motion at the moment of positioning, and thus it is possible to use two grating rulers of different capacities to measure displacement, which include one coarse grating ruler for high-speed, low-definition measurement and one precise grating ruler for low-speed, high-definition measurement. When the positioning system has its motion shift to low speed form high speed, the method switch the counting pulses of the two grating rulers and synthesizes their counts, so as to obtain high-definition displacement value at the moment of positioning. In one of its specific counting schemes, a threshold of speed for switching is set. For high-speed motion, the coarse grating ruler measures displacement and speed. When the motion speed becomes below the set threshold, the precise grating ruler takes over for motion displacement. While the related art achieves high-speed, high-precision displacement measurement using grating rulers, it has some shortcomings: 1 it is unable to provide high-speed, high-precision displacement measurement throughout the whole course of motion, so is less applicable; and 2. it uses two grating rulers and thus requires more costs.